Heretofore, detectors of combustion products which incorporate ion chambers occasionally generated undesirable or false alarm signals. These unwanted signals indicated that a relatively minute percentage of combustion products was present in the atmosphere. This small percentage resulted from sources such as particles of combustion from a cigarette or the combustion products emitted during the normal process of cooking food. In addition to the relatively low percentage of combustion products generating a flase alarm, other noncombustion sources, such as high air velocity or high humidity, would occasionally generate a false alarm. Since it is not desirable to generate an alarm under these types of normal conditions, the ion chamber could be set so that a relatively greater percentage of combustion products would be required before the alarm is triggered. However, this approach is unsatisfactory since it is essential that combustion products resulting from a smoldering fire be detected. Since a smoldering fire emits a relatively small concentration of combustion products, its presence would not be detected by an ion chamber set to generate an alarm only when a relatively greater concentration of combustion products is present. Therefore, to permit the ion chamber to generate an alarm when a relatively small percentage of combustion products is present in the atmosphere and still avoid an undesirable alarm resulting from the presence of conditions such as increased air velocity or high humidity, the invention described herein is provided.